Canker sores? I never had that problem ... but I have picked up an occasional upper resperatory infection after playing harmonica - on a few occasions. I play hard and draw a lot of air, and probably contaminents against my throat and larynx ... I believe the minor irriations caused by that 'spray' can sometimes create a welcome home for bacteria. I have found that if I feel that 'tickle' in my throat after playing that if I gargle with an anti-bacterial mouth wash - an infection never takes root.